1. Field of the Invention
The present invention relates to heat insulation walls requiring heat insulation in a heat insulation box body such as a refrigerator, or the like, in which wall surfaces are formed of thin metal plates, resin moldings, or the like. More particularly, the present invention relates to a full vacuum heat insulation box body in which porous structural materials are disposed in a shell constituting heat insulation walls for the purpose of preventing deformation so that a vacuum is kept, a refrigerator using such a full vacuum heat insulation box body, a method for producing such a full vacuum heat insulation box body, and a method for disassembling such a full vacuum heat insulation box body.
2. Description of the Related Art
Conventionally, a shell of a refrigerator, or the like, is so constituted that an outer box is formed of a thin metal plate such as an iron plate, an inner box is formed of a resin molding, and closed-cell foaming urethane used for forming a structural material, is injected into a gap between the inner and outer boxes and foamed so that the gap is filled with the structural material.
FIG. 16 is a flow chart illustrating a process of producing a conventional refrigerator using closed-cell foaming urethane as a heat insulating material in walls, and FIG. 17 illustrates a foaming urethane injection step in the process.
That is, in a conventional refrigerator, or the like, an inner box 2 obtained by attaching necessary members, such as an anchor for fixing interior parts, piping for supplying a refrigerant, etc., to a vacuum molding of an ABS resin sheet, is inserted in an outer box 1 of a formed product obtained by bending a steel plate to thereby form a shell. Injection portions 4 are provided in the outer box 1 (step 1) to inject a mixture solution 3 of foaming urethane.
After sheet metal worked parts are attached to the back and bottom portions which are residual opening portions, a slight gap in each engaging portion is sealed with a hot melt adhesive agent, or the like, and further interior parts are partially assembled (step 2).
The thus obtained box body is laid down as shown in FIG. 17, and fixed in a foaming jig heated to an arbitrary temperature. After a mixing head 5 is successively inserted into and fixed to injection holes of the injection portions 4 provided in the outer box 1, a mixture solution 3 of foaming urethane is discharged and injected. Then, injection portions 4 are sealed with plugs. Because the foaming urethane mixture solution 3, at the time of injection, is a liquid having an expansion ratio in a range from several times to tens of times, the mixture solution 3 flows in a flange portion corresponding to the opening portions of the box body through the injection portion 4 so as to disperse. Further, after some seconds, a foaming agent is vaporized by reaction heat of raw materials and thereby the foam is caused to fill the residual gap between the inner box 2 and the outer box 1 with urethane foam. A heat insulation box body thus formed can be taken out from the foaming jig after some minutes, generally about 5 minutes from the injection (step 3).
Residual parts, for example, electric parts such as a fan motor and a light and interior parts such as shelves and various kinds of casings are put in the thus obtained heat insulation box body. After refrigerant circuit securing parts for securing a refrigerant circuit are attached to the heat insulation box body, the refrigerant circuit is charged with a refrigerant. Thus, assembling of the product is completed (step 4).
Inspection of various kinds of functions of the completed product is carried out through an actual operation so as to confirm that the product is not defective (step 5).
When a package and documents pertinent to the obtained product are prepared and added, the production is completed (step 6).
It has been found that the chlorine containing 1,1-dichloro-1-fluoroethane (HFC141b), which is one of hydrochlorofluorocarbons that has been used as a foaming agent for forming urethane foam used as a heat insulating material herein, is a cause of ozone layer destruction. Accordingly, use of hydrofluorocarbons or hydrocarbons which do not contain chlorine in their molecules, has been proposed in recent years.
For example, a method for producing urethane foam by use of hydrofluorocarbons such as 1,1,1,3,3-pentafluoropropane (HFC245fa) and 1,1,1,4,4,4-hexafluorobutane (HFC356mffm) as a foaming agent is disclosed in JP-A-2-235982, and a method for producing urethane foam by use of hydrocarbon such as cyclopentane, or the like, as a foaming agent is disclosed in JP-A-3-152160.
However, the heat insulating property of such urethane foam is in a range from 19 to 20 mw/MK and clearly inferior to the heat insulating property of 16 mw/MK of chlorofluorocarbons used before issue of regulations on use of ozone layer destruction substances.
Since the improvement of the heat insulating property of urethane foam has reached a limit, a technique of applying a vacuum heat insulation panel which has more than twice as higher heat insulating property as the urethane foam as shown in the comparison view of FIG. 18 has been proposed for a refrigerator, or the like, allowing a reduction of electric power consumption without use of any substance which causes ozone layer destruction.
For example, JP-A-60-243471 discloses a heat insulation box body in which a member obtained by putting pulverized PUF in a synthetic resin bag and vacuum-packing the pulverized PUF in the form of a board is disposed inside walls, and JP-A-60-60483 proposes a refrigerator in which a vacuum heat insulation panel having a gap which is provided in the flange side of a side plate to allow PUF to flow in the gap is disposed in a side wall of the refrigerator.
The vacuum heat insulation panel such as those proposed above, has a structure shown in FIG. 19. A method for producing the vacuum heat insulation panel will be described below. First, a core material 11 having a porous structure such as an aggregate of fibers or particles, a foam having open cells, or the like, is inserted into a bag-like packing material 12. Then, in order to generate a high quality heat insulating property, its inside is deaerated by using a vacuum panel making machine 15 comprising fusion-bonding devices 17 each having a heater 17a, sealing pressure devices 18, and a vacuum control valve 16 as shown in FIG. 20. While a vacuum state is maintained, end edge portions 12a of the packing material 12 containing the core material 11 are heat-sealed to prevent external air from entering inside. Thus, a vacuum heat insulation panel 13 shown in FIG. 19 is obtained. Preferably, the inside of the vacuum panel making machine 15 is kept to 10.sup.-2 torr when the end edge portions 12a are subjected to fusion bonding. Therefore, adjustment of the degree of vacuum is performed by use of the vacuum control valve 16 connected to an evacuator not shown.
Accordingly, in the packing material 12, a thin metal film layer is used as its intermediate layer for blocking or suppressing entrance of gas from the outside into the vacuum heat insulation panel to thereby keep a heat insulating property. A material having excellent welding property is used as its inner layer so that insertion openings can be sealed perfectly, and a material for stably securing adhesion to urethane foam is used as its surface layer so that generation of scratches is suppressed and bending strength of walls in a box body such as a refrigerator, or the like, can be secured. Because the packing material 12 is required to have various characteristics as described above, a multilayer sheet in which different materials are laminated to satisfy the required characteristics is used.
Further, the core material 11 must have a strength higher than atmospheric pressure to satisfy a function of holding the panel shape in a vacuum state and the quantities of conducted heat (heat conduction) and penetrated heat (heat radiation) through a substance constituting the core material itself must be suppressed to thereby contribute to improvement of heat insulating property. Accordingly, a porous plate formed of a substance with small heat transfer rate is used as the core material 11.
That is, in order to improve the heat insulating property of the vacuum heat insulating panel 13, it is important to use a substance that is a good insulator for the core material 11 among constituent materials, reduces the heat-conduction area of the material to suppress the heat conduction through the substance, and reduces the gap to suppress heat radiation. As a substance satisfying the aforementioned conditions, a porous material of resin, glass, or the like, is preferably used. In particular, a mat of glass fiber, a board of a resin foam having open cells, or a molding of resin or inorganic fine particles is used preferably.
For example, JP-A-60-71881 has proposed a material obtained by putting pearlite powder in a synthetic resin bag and vacuum-packing it into the form of a board. Similarly, JP-A-60-243471 has proposed a material obtained by putting pulverized PUF in a synthetic resin bag and vacuum-packing it into the form of a board. As other proposals, JP-A-60-205164 has proposed hard polyurethane foam having open cells, JP-A-4-218540 has proposed a plate-like molding which is formed from thermoplastic urethane resin powder firmly bonded and, JP-A-7-96580 has proposed a board which comprises long glass fiber, fibrillated resin fiber and inorganic fine powder, each of which is applied as a core material of the vacuum heat insulation panel.
Each of the vacuum heat insulation panels, such as those proposed above, is generally shaped as a board or a substrate having a thickness in a range from 10 to 20 mm and is typically incorporated into the wall of the refrigerator. That is, after the inner box is inserted into the outer box equipped with the vacuum heat insulation panels stuck thereon so that the inner box is united with the outer box, a raw material mixture solution of foaming urethane is injected thereto, foamed and molded to thereby form a heat insulation wall.
Accordingly, in the case of a refrigerator, the vacuum heat insulation panel is usually not stuck on the inner box having an uneven surface for shelf rests, or the like, but fixed to the outer box surface by use of an adhesive agent, or the like, so that foaming urethane to fill the gap in the shell containing the vacuum heat insulation panels disposed therein is fully packed without any remaining gap to thereby prevent spoilage of design characteristic such as deformation, or the like.
However, in the cases that the packing material has some fine defect which is larger than expected, a part of the packing material is destroyed by an external factor or a large amount of volatile substance remains in or sticks to the core material, thereby creating a number of possibilities that a desired heat insulating property cannot be provided.
As described above, in the heat insulation wall structure of the conventional heat insulation box body, the vacuum heat insulation panel is disposed in the shell and the residual space is filled with urethane foam having closed cells. Therefore, if the aforementioned failure occurs in the vacuum heat insulation panel, it is not only very difficult to repair the vacuum heat insulation panel but also impossible to replace the vacuum heat insulation panel with a new one. That is, the heat insulation wall is conventionally formed on the assumption that the whole of a system such as a heat insulation box body, a refrigerator, or the like, must be scrapped when the aforementioned failure occurs.
As a method to enable lowering of the degree of vacuum caused by the aforementioned possibilities to be repaired, there has been proposed a heat insulation box body having heat insulation walls in which all the inside of the shell of the heat insulation box body is set in a vacuum state. For example, JP-A-57-52783 has proposed to insert an air-permeable bag containing a powder substance into the gap between the inner and outer boxes, JP-A-3-140782 has proposed to put particles of pearlite, or the like, into the hollow resin shell, and JP-A-2-192580 and JP-A-7-148752 have proposed to inject foaming heat insulating material such as foaming urethane with open cells into the shell. Each of the shells is evacuated with a vacuum pump, or the like, through a gas exhaust hole provided in a part of the shell to secure the vacuum state inside the shell of the heat insulation box body.
In the conventional heat insulation box body configured so that all the heat insulation wall is kept in a vacuum state as described above, it has been found that it is very difficult to fill the inside of the shell with a powder or granular substance uniformly and densely when the powder or granular substance is put in the shell. Accordingly, if the inside of the shell is kept in a vacuum state, the shell is pressed by atmospheric pressure so as to be partly or wholly contracted, so that deterioration of design characteristic may be caused or in some cases, deterioration of heat insulating property caused by reduction of the wall thickness may be triggered.
Further, in filling a heat insulation box body having inferior filling property such as a large-size refrigerator, or the like, a larger amount of filling is required than the amount of filling corresponding to the density for obtaining a strength required to prevent deformation caused by the atmospheric pressure.
Accordingly, there arise disadvantages such as economical loss, increase of weight, lowering of heat insulating property, etc.
Further, in filling the heat insulation box body with open-cell foaming urethane, communication of bubbles cannot be sufficiently achieved so that closed cells remain, if bubbles in a foamed state flow over a short distance from the start point of foaming, bubbles flow in a state of stable shape after completion of bubble growth, and so on.
Further, because foaming gas remaining in bubbles remains in cells or is adsorbed into a resin constituting cells even in a portion in which communication of cells is achieved, foaming gas remains. Accordingly, if this is used as it is, for a structural material, there arises a disadvantage that not only a long time is required for evacuation particularly of a large-size full vacuum heat insulation box body but also a degree of vacuum changes is lost over the passage of time.
That is, in accordance with the aforementioned proposals, it is indispensable to perform troublesome evacuation substantially periodically by use of a vacuum pump, or the like, or to incorporate a suction system for the purpose of preventing a drop in the degree of vacuum due to generation of gas in the shell. Furthermore, in a state where the inside of the shell is filled with no gap, a long evacuation time is required because this structure brings a great disadvantage for sucking remaining gas in an opposite portion inside the shell to the gas-exhaust hole up to all open cells through a long distance along open cells by use of a vacuum pump from a gas-exhaust hole provided in an end portion of the heat insulation box body such as a refrigerator, or the like, to thereby perform evacuation to secure a sufficient vacuum state. Further, during the period when the degree of vacuum drops with the passage of time, a cooling operation is carried out frequently, so that electric power is additionally consumed and the temperature of the inside of the refrigerator becomes unstable to cause a problem in that the freshness of foods is affected.
Further, when the full vacuum heat insulation box body obtained by the conventional production method is to be disassembled after scrapping so as to recycle parts or members, some measures are required to prevent scattering of the filling materials at the time of disassembling or collecting in the former case of filling powder or granular materials, and it is also difficult to handle the materials without damage even in the case of employing a method in which the filling materials are disposed in a form protected by bags, or the like.
On the other hand, in the latter case of the full vacuum heat insulation box body in which a raw-material mixture solution of foaming urethane is injected into the shell and foamed to thereby form heat insulation walls, the filled urethane foam firmly self-adheres to the inner and outer boxes constituting the shell so as to be nearly inseparable therefrom when the box body is to be disassembled after scrapping to recycle the members. In the conventional method therefore, the shell is not separated into constituent members but the inner and outer boxes and the filled urethane foam self-adhering thereto are collectively subjected to a crusher so as to be broken up, and then, the crushed parts are separated into respective members by use of a separation method using weight or magnetic characteristic arranged for a subsequent step to the crusher, so that the outer box is magnetically attached, the inner box is made to fall down by itself by weight and the urethane foam is flown off, for example, laterally by use of wind, or the like. It is however impossible to perfectly separate the urethane foam self-adhering to the inner and outer boxes from adhering surfaces. Accordingly, used members cannot be reused and therefore, recycling of the members is difficult using the conventional methods.